Is Space In The Brain Limited?

Is space in the brain limited? As we get older, must we forget some things in order to "make space" for new memories and connections between memories? A study published in Nature suggests that this might be the case.

Working with mice, [Cornelius Gross, who led the work at European Molecular Biology Laboratory] and colleagues studied the hippocampus, a region of the brain known to help form memories. Information enters this part of the brain through three different routes. As memories are formed, connections between neurons along the “main” route become stronger...

Gross proposes that one explanation: “There is limited space in the brain, so when you’re learning, you have to weaken some connections to make room for others,” says Gross.

Interestingly, this active push for forgetting only happens in learning situations. When the scientists blocked the main route into the hippocampus under other circumstances, the strength of its connections remained unaltered.

The findings were made using genetically engineered mice, but the scientists demonstrated that it is possible to produce a drug that activates this “forgetting” route in the brain without the need for genetic engineering. This approach, they say, might help people forget traumatic experiences.

The first time I encountered this idea was in Arthur Conan Doyle's 1887 novel A Study in Scarlet:

“I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skillful workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”

Another reference to this problem, with some ideas about how it happens, is found in Robert Heinlein's 1941 story Methuselah's Children. In the story, Lazarus Long was worried about whether or not an effectively immortal human being would start to run out of memory space:

...Lazarus frowned slightly. "Funny thing, Andy... I recall that vividly, I've always had a good memory-yet it seems to be getting harder for me to keep things straight. Especially this last century."

"Inescapable mathematical necessity," said Libby.

"Huh? Why?"

"Life experience is linearly additive, but the correlation of memory impressions is an unlimited expansion. If mankind lived as long as a thousand years, it would be necessary to invent some totally different method of memory association in order to be eclectively time-binding. A man would otherwise flounder helplessly in the wealth of his own knowledge, unable to evaluate. Insanity, or feeble-mindedness."

Human beings may need to have some sort of additional storage to make sure memories are available. Heinlein had some ideas on this, too, in Time Enough For Love, which continues the story of Lazarus Long, a very long-lived character indeed.

"I can use it to write the sort of commentary you want, and take time to miniaturize and stabilize a message. The problems of a time-tripping historiogapher are odd and awkward. One Welton fine-grain memory cube would record all I could say over the next ten years--except that I would have no use for one even if I had it; the technology to use it is lacking.

"By to way--Ishtar, did you plant a recorder in my belly? You are a darling, dear, but sometimes a devious darling--and there is something there.

... But I suspect that it is a Welton cube with an ear hooked to it and a ten-year power supply; it is about the right size."
(Read more about Welton cube